In metropolitan areas it is not uncommon to run communications cable in underground ducts which are located adjacent to steam lines. The operating condition of the steam varies generally from 0.7 MPa (100 psig) to 2.8 MPa (400 psig). The corresponding minimum temperature of the steam, which is higher if superheated, varies from 165.degree. C. (328.degree. F.) to 230.degree. C. (445.degree. F.).
The leaking of underground steam piping is of great concern to telecommunication operating companies. This is particularly true inasmuch as underground steam ducts in many cities may be as old as 100 years and prone to leakage because of aging and fatigue. Optical fiber cables placed in the neighborhood of steam pipes are vulnerable to damage and failure when there is a steam leak. When a leak in a high pressure steam pipe occurs, adjacent optical fiber cable is exposed to a high temperature, high moisture, and high velocity environment. The values, of course, depend on the distance between the location of the leak in the steam pipe and the cable. It is estimated from field data that the maximum steam temperature to which the cables may be exposed when there is a steam pipe leak could be as high as 140.degree. C. (284.degree. F.). The duration of exposure could be as long as a few years. Currently available commercial optical fiber cables have been found to fail in such an environment. The failures are not surprising because cables presently made were not designed for such an environment. The high temperature, high moisture, and high velocity environment causes cable materials to degrade or melt, resulting in the failure of optical fibers over a period of time.
Because steam may have such adverse effects, it becomes necessary for a cable adjacent to steam lines to have a sheath system which is capable of preventing damage to optical fiber when the cable is exposed to escaping steam. Important considerations for a steam-resistant optical fiber cable are high temperature and high moisture conditions. These conditions relate directly to the basic high temperature and hydrolytic stability performance of the materials of a sheath system of the cable. The high velocity condition is more closely related to the structural integrity and mechanical performance of the cable, and should be used as a secondary design consideration of the cable along with the cable racking condition, that is, how the cable is supported along its length.
When a leak occurs in an adjacent steam line, escaping steam can cause a portion of the cable to be exposed to a high temperature. For a conventional non-hermetically sealed cable, unlimited oxygen and moisture from the atmosphere can reach the cable materials at the steam line leak. The cable materials at that location are exposed to humidity and oxygen in a high temperature environment. A high temperature steam environment is conducive to hydrolysis and a high moisture condition hydrolizes many thermoplastic materials. Such hydrolytic stability degradations are accelerated significantly as the temperature is increased. On the other hand, the high temperature condition alone without a high moisture condition and oxygen environment may be less detrimental to cable materials, especially those materials which are thermally stable at high temperatures in the absence of moisture and oxygen.
A high temperature steam environment also is conducive to the hydrolysis of acrylate materials. Acrylate materials typically are used to coat drawn optical fiber. Optical fibers with conventional acrylate coatings are capable of sustained performance at relatively high temperatures, but not in the presence of the high moisture or oxygen. Thus, it becomes imperative that critical components of a steam-resistant cable be protected from the combination of a high temperature and high humidity and/or oxygen in order for the cable to perform satisfactorily for a substantial period of years.
In the past, polyethylene-jacketed, lead-shielded cables which are relatively expensive were used in steam environments. Because it tends to develop cracks or melt when exposed to high temperatures for a long period of time, the polyethylene jacket is sacrificial only and the cable relies on inner portions of its sheath system for sustained performance in a steam environment. Cables having a polyethylene jacket extruded over a soldered seam steel shield also have been used. However, in cables of this latter construction, the soldered seam generally has not been continuous.
In one optical fiber cable suitable for use adjacent to steam piping, the cable includes a core comprising at least one optical fiber transmission medium and a first tubular member in which is disposed the core and which comprises a plastic material that resists degradation when exposed to relatively high temperatures. A second plastic tubular member which is referred to as an inner jacket is disposed about the first tubular member and comprises a plastic material. Interposed between the first and second tubular members is a strength member system. Disposed about the second tubular member is a hermetic sealing member which comprises a metallic material having a relatively low chemical or electrochemical reactivity. An outer jacket comprising a plastic material is disposed about the hermetic sealing member. In a preferred embodiment, a filling composition of matter is disposed in the core to provide a waterblocking function. Also, in the preferred embodiment, a waterblocking member such as a yarn or tape which includes a superabsorbent polymeric material is interposed between the first and the second tubular members. See application Ser. No. 07/785,602 filed on Oct. 30, 1991 now allowed, in the names of K. Kathiresan, A. J. Panuska and M. R. Santana.
The last-mentioned hermetically sealed cable structure provides protection for the transmission media from the circumferential entry of oxygen and humidity at the location of a leak in a steam line. However, the hermetic seal does not prevent the limited diffusion of oxygen and moisture longitudinally along the cable.
In a hermetically sealed environment, it has been found that materials which appear to be useful in a cable exposed to a steam environment have been found to degrade whereas those that may be eliminated at first thought may turn out to be effective. Indeed it has been found that particular core tube materials have darkened, indicating degradation, in the presence of particular filling compositions whereas others do not.
What is needed and what seemingly does not appear to be available in the prior art is a hermetically sealed optical fiber cable which is capable of providing reliable transmission for a substantial period of time notwithstanding exposure to a high temperature, high humidity and high velocity steam environment. Of course, such an optical fiber cable must not have an unduly large diameter and must be reasonable in cost.